The present invention relates to mass spectrometers.
A mass spectrometer analyzes masses of particles, such as atoms and molecules, and typically includes an ion source, one or more mass analyzers and detectors. In the ion source, particles are ionized and extracted from a sample. The particles can be ionized using a variety of techniques, such as electrostatic forces, or laser, electron, or other particle beams, and the ions can be extracted using electric fields. The ions are transported to one or more mass analyzers that separate the ions based on their mass-to-charge ratio. The separated ions are detected by one or more detectors that provide data that is used to construct a mass spectrum of the sample.
The ions can be guided, trapped, and analyzed by multipole rod assemblies, including but not limited to quadrupole, hexapole, octapole or greater assemblies including four, six, eight, or more multipole rods, respectively. (Techniques for preparing such assemblies are described, for example, in U.S. Pat. No. 5,389,785 to Steiner et al, filed Apr. 28, 1993, which is incorporated by reference herein in its entirety.) In the assembly, the multipole rods are arranged to define an interior volume, e.g., a channel or a ring, in which multipole electric potentials can be generated by applying voltage on the multipole rods. For example, quadrupole electric potentials can be generated in a quadrupole rod assembly including two pairs of opposing rods by applying a voltage on the first pair and an inverse voltage on the second pair. By periodically changing the applied voltage, the quadrupole electric potentials can guide or trap in the interior volume ions that have mass-to-charge ratios within an effective range. The effective range is defined by mass-to-charge ratios of ions that can be guided or trapped in the interior volume. Ions with mass-to-charge ratios outside the effective range escape the interior volume.
The effective range of mass-to-charge ratios can be tuned by the applied voltage and its frequency. For guiding or trapping ions, the effective range is typically kept wide. For analyzing the guided or trapped ions, the effective range can be narrowed such that only ions with particular mass-to-charge ratios leave the interior volume. These ions can be detected to measure a mass spectrum. Resolution of the measured spectrum depends on the precision of the multipole electric potentials that, in turn, depend on the shape and position of the multipole rods in the assembly.